全文获取类型
收费全文 | 49772篇 |
免费 | 5084篇 |
国内免费 | 3122篇 |
专业分类
电工技术 | 1615篇 |
综合类 | 2869篇 |
化学工业 | 10764篇 |
金属工艺 | 7709篇 |
机械仪表 | 3537篇 |
建筑科学 | 1171篇 |
矿业工程 | 1160篇 |
能源动力 | 2147篇 |
轻工业 | 2721篇 |
水利工程 | 365篇 |
石油天然气 | 1108篇 |
武器工业 | 303篇 |
无线电 | 6320篇 |
一般工业技术 | 11188篇 |
冶金工业 | 3143篇 |
原子能技术 | 684篇 |
自动化技术 | 1174篇 |
出版年
2024年 | 104篇 |
2023年 | 1085篇 |
2022年 | 1140篇 |
2021年 | 1598篇 |
2020年 | 1768篇 |
2019年 | 1642篇 |
2018年 | 1413篇 |
2017年 | 1778篇 |
2016年 | 1654篇 |
2015年 | 1666篇 |
2014年 | 2406篇 |
2013年 | 2884篇 |
2012年 | 3114篇 |
2011年 | 3585篇 |
2010年 | 2589篇 |
2009年 | 2796篇 |
2008年 | 2476篇 |
2007年 | 3237篇 |
2006年 | 3108篇 |
2005年 | 2728篇 |
2004年 | 2341篇 |
2003年 | 2173篇 |
2002年 | 1760篇 |
2001年 | 1542篇 |
2000年 | 1357篇 |
1999年 | 1082篇 |
1998年 | 943篇 |
1997年 | 716篇 |
1996年 | 658篇 |
1995年 | 561篇 |
1994年 | 500篇 |
1993年 | 404篇 |
1992年 | 288篇 |
1991年 | 217篇 |
1990年 | 162篇 |
1989年 | 150篇 |
1988年 | 89篇 |
1987年 | 56篇 |
1986年 | 31篇 |
1985年 | 33篇 |
1984年 | 32篇 |
1983年 | 26篇 |
1982年 | 21篇 |
1981年 | 11篇 |
1980年 | 8篇 |
1979年 | 8篇 |
1977年 | 4篇 |
1975年 | 6篇 |
1959年 | 5篇 |
1951年 | 7篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
41.
42.
43.
The sun and outer space are the ultimate heat and cold sources for the earth, respectively. They have significant potential for renewable energy harvesting. In this paper, a spectrally selective surface structure that has a planar polydimethylsiloxane layer covering a solar absorber is conceptually proposed and optically designed for the combination of photothermic conversion (PT) and nighttime radiative sky cooling (RC). An optical simulation is conducted whose result shows that the designed surface structure (i.e., PT-RC surface structure) has a strong solar absorption coefficient of 0.92 and simultaneously emits as a mid-infrared spectral-selective emitter with an average emissivity of 0.84 within the atmospheric window. A thermal analysis prediction reveals that the designed PT-RC surface structure can be heated to 79.1°C higher than the ambient temperature in the daytime and passively cooled below the ambient temperature of approximately 10°C in the nighttime, indicating that the designed PT-RC surface structure has the potential for integrated PT conversion and nighttime RC utilization. 相似文献
44.
Jinmei Li Wei Wang Fengxia Wang Yumao Kang Ting Tan Ziqiang Lei 《International Journal of Hydrogen Energy》2018,43(2):551-561
The development of biomass-derived nitrogen-doped porous carbons (NPCs) for the oxygen reduction reaction (ORR) is important for sustainable energy systems. Herein, NPCs derived from Astragali Radix (AR) via a cost-effective strategy are reported for the first time. The as-prepared AR-950-5 catalyst shows a stacked layer-like structure and porosity. Notably, the optimized AR-950-5 delivers catalytic activity comparable to that of commercial Pt/C (C-Pt/C), with high onset potential, positive half-wave potential and large limiting current density. It also displays superior long-term stability and methanol tolerance for ORR. This work will pave the way for a new approach in the development of highly active and low-cost NPCs for fuel cells. 相似文献
45.
Woohyun J. Moon Prof. Dr. Juewen Liu 《Chembiochem : a European journal of chemical biology》2020,21(3):401-407
It has been proposed that Mg2+ and Fe2+ are very similar in interacting with ribozymes and some protein-based enzymes, but their activities with DNAzymes have yet to be studied. Here, the activity of Fe2+ as cofactor for a few RNA-cleaving DNAzymes is investigated. 17E is a well-studied DNAzyme that is active in the presence of many different divalent metal ions; it is highly active with Fe2+ with an apparent Kd of 29.7±2.3 μm and a kobs of 1.12±0.11 min−1 in the presence of 1 mm Fe2+ at pH 7.5. Fe2+ has 21-fold higher activity than Mg2+. Six different DNAzymes are then tested, and only the DNAzymes active with Mg2+ (17E, 8–17, and E5) are active with Fe2+. Fe2+ has 25 and one- to twofold higher activity than Mg2+ for the 8–17 and E5 DNAzymes, respectively. In pH>7 buffer and in presence of air, 1 mm Fe2+ results in a nonspecific degradation of the DNA strand due to reactive oxygen species (ROS). Cleavage reactions in anoxic environment and antioxidant ascorbate can be used to overcome the effect of oxidation. The findings provide insights for potential DNAzyme catalysis in the early Earth, and they further support the similarity between Mg2+ and Fe2+ in enzyme catalysis. 相似文献
46.
为探索微观表面形貌对液膜密封空化的影响,基于满足质量守恒的JFO空化模型及坐标变换,建立考虑微观表面形貌的双坝区螺旋槽液膜密封数学模型;采用有限体积法离散求解控制方程,综合分析表面粗糙度、周向波度和径向锥度对螺旋槽液膜密封空化发生的影响规律。结果表明:相比而言,密封面计入微观表面形貌后,摩擦副液膜中空穴区发生位置分散且形状不规则;以空化面积比为判据,较大表面粗糙度对液膜空化促生虽起到积极作用,但数据较小,可忽略不计;锥度对液膜中空穴促生和抑制影响有限,波幅的增加显著促进液膜中空穴的发生;高频波数时,正锥度有利于降低液膜空化面积比,抑制空穴。 相似文献
47.
根据角接触球轴承自旋运动特征,同时考虑弹流润滑效应,建立角接触球轴承考虑自旋运动的弹流润滑模型;采用多重网格法求解弹性变形,利用有限差分法迭代求解雷诺方程,得到较为精确的数值解;分析不同赫兹接触压力、滚道表面粗糙度下自旋对角接触球轴承弹流润滑和油膜刚度的影响。结果表明:考虑自旋时随着Hertz接触压力、自旋角速度增大,油膜厚度减小,油膜压力增大,油膜承压区域呈细长状,并向接触中心靠近;随着滚道表面粗糙度幅值增大,油膜压力和膜厚均出现了波动,且考虑自旋运动时,轴承油膜厚度明显减小,油膜局部压力峰值更大;随着卷吸速度、润滑油黏度增大,油膜刚度减小,而考虑自旋运动时油膜刚度值更大;随着自旋角速度增大,油膜刚度逐渐增大。 相似文献
48.
Milinda Pattanayak Md Nadim F Hoque Zhaoyang Fan Ayrton A. Bernussi 《Science and Technology of Advanced Materials》2018,19(1):693-701
We report large amplitude modulation waveforms as large as ~ 10 V using vanadium dioxide micro-channel devices operating under current-controlled conditions. The self-sustained electrical oscillations were generated by controlling the applied current in the negative differential resistance region of the investigated devices. An appropriate value of internal capacitance was achieved as parasitic capacitance in the device structure to stabilize the electrical oscillations. This eliminates the need of an external pulsed source or any external passive component connected to the micro-channel devices. Amplitude and frequency of the oscillation were tuned by illuminating the device micro-channel with an external laser. An equivalent circuit model was developed to simulate the waveforms. A good agreement between experiment and simulation was verified. 相似文献
49.
Chengyong Yang Bolong Huang Shuxing Bai Yonggang Feng Qi Shao Xiaoqing Huang 《Advanced materials (Deerfield Beach, Fla.)》2020,32(24):2001267
Electrocatalytic nitrogen reduction reaction (NRR) is a promising process relative to energy-intensive Haber–Bosch process. While conventional electrocatalysts underperform with sluggish paths, achieving dissociation of N2 brings the key challenge for enhancing NRR. This study proposes an effective surface chalcogenation strategy to improve the NRR performance of pristine metal nanocrystals (NCs). Surprisingly, the NH3 yield and Faraday efficiency (FE) (175.6 ± 23.6 mg h–1 g–1Rh and 13.3 ± 0.4%) of Rh-Se NCs is significantly enhanced by 16 and 15 times, respectively. Detailed investigations show that the superior activity and high FE are attributed to the effect of surface chalcogenation, which not only can decrease the apparent activation energy, but also inhibit the occurrence of the hydrogen evolution reaction (HER) process. Theoretical calculations reveal that the strong interface strain effect within core@shell system induces a critical redox inversion, resulting in a rather low valence state of Rh and Se surface sites. Such strong correlation indicates an efficient electron-transfer minimizing NRR barrier. Significantly, the surface chalcogenation strategy is general, which can extend to create other NRR metal electrocatalysts with enhanced performance. This strategy open a new avenue for future NH3 production for breakthrough in the bottleneck of NRR. 相似文献
50.
Zhonghao XIE 《等离子体科学和技术》2020,22(8):85503
Tegillarca granosa (T. granosa) is susceptible to heavy metals, which may pose a threat to
consumer health. Thus, healthy and polluted T. granosa should be distinguished quickly. This
study aimed to rapidly identify heavy metal pollution by using laser-induced breakdown
spectroscopy (LIBS) coupled with linear regression classification (LRC). Five types of T.
granosa were studied, namely, Cd-, Zn-, Pb-contaminated, mixed contaminated, and control
samples. Threshold method was applied to extract the significant variables from LIBS spectra.
Then, LRC was used to classify the different types of T. granosa. Other classification models
and feature selection methods were used for comparison. LRC was the best model, achieving an
accuracy of 90.67%. Results indicated that LIBS combined with LRC is effective and feasible
for T. granosa heavy metal detection. 相似文献